The present invention relates to a process for the anaerobic production of acetic acid from substrates for homoacetogenic bacteria, in particular from sugar solutions, by fermentation using homoacetogenic bacteria in an aqueous medium.
Acetic acid is a raw material of importance to the chemical industry that is required in increasing amounts and that is presently prepared, in large quantities, from petroleum and natural gas. Acetic acid can also be prepared by microbial conversion of sugars by (1) alcoholic fermentation and then (2) incomplete oxidation of the resulting ethanol by aerobic acetic-acid bacteria, as briefly outlined below: EQU C.sub.6 H.sub.12 O.sub.6 (glucose) .fwdarw.2 CH.sub.3 CH.sub.2 OH+2 CO.sub.2 EQU 2 CH.sub.3 CH.sub.2 OH+2 O.sub.2 .fwdarw.2 CH.sub.3 COOH+2 H.sub.2 O.
In this two-step conversion of glucose into acetic acid, a maximum of 0.67 kg of acetic acid is formed per 1 kg of glucose, i.e., the maximum possible yield based on reacted glucose is only 67%. In contrast, it is possible using anaerobic homoacetogenic bacteria to convert sugars virtually quantitatively into acetic acid: EQU C.sub.6 H.sub.12 O.sub.6 .fwdarw.3 CH.sub.3 COOH.
The latter process is called "homoacetate fermentation." Bacteria that are "homoacetogenic," i.e., that live strictly anaerobically and are capable of converting glucose, fructose or xylose into acetic acid via homoacetogenic fermentation, have been known for some time. For example, a few strains of the genera Clostridium, Acetobacterium and Acetooenium, respectively, are able to convert not only sugars but also other low molecular weight compounds, such as lactate, pyruvate, carbon monoxide, formate, carbon dioxide and hydrogen, and methanol, into acetic acid.
The ability to carry out the quantitative conversion of sugars or other substrates into acetic acid in a single step, while dispensing with the introduction of oxygen into the bacterial suspension which is necessary in aerobic processes, makes homoacetate fermentation appear particularly favorable. But there is a considerable disadvantage in homoacetate fermentation presented by the sensitivity of known homoacetogenic bacteria to acetic acid, resulting in pronounced inhibition of acetic-acid formation as the concentration of the fermentation product increases.
It is normally impossible, therefore, to reach acetic acid concentrations above about 3.5%. The low acetic-acid contents achieved with conventional homoacetate fermentations processes have hindered the economical use of such processes.